Award Dates: December 18, 2017 — December 17, 2018
Characterizing the deep biosphere methane cycle is of high importance in light of human alterations to the global carbon cycle. Subseafloor reservoirs of methane have been significant in past climate systems and may soon be quite significant for our present climate. Below the seafloor, multitudes of microbes carry out enigmatic metabolisms. Methane eaters (the ANaerobic MEthanotrophs, or ANME) are suspected to consume ~90% of the methane produced by other deep biosphere microbes. Without this microbial methane filter, fluxes of methane to the atmosphere from the oceans could be much larger. Despite its importance, much about this crucial process remains enigmatic. I work with new tools potentially capable of offering new insights into the subseafloor methane cycle. These techniques, broadly known as “clumped-isotope” approaches, focus on the abundance of methane molecules that have more than one rare isotope substitution. I propose the use of methane clumped-isotope geochemistry to distinguish methane production from consumption and independently constrain the rates of these processes. By measuring the methane clumped-isotope compositions of methane from lab cultures and well-characterized natural samples, I aim to build an isotopologue mass-balance model that can better quantify deep biosphere methane cycling in a variety of environments.
We report measurements of resolved 12CH2D2 and 13CH3D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve 12CH2D2 from 13CH3D provides unprecedented insights into the origin and evolution of CH4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Δ12CH2D2 and Δ13CH3D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH4/H2 D/H exchange, underscoring the importance of reliable thermometry based on the CH4 molecules themselves. Where Δ12CH2D2 and Δ13CH3D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis vs. biological processes. Deficits in 12CH2D2 compared with equilibrium values in CH4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in 13CH3D that accompany the low 12CH2D2 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ12CH2D2 values are a key tracer of microbial recycling.